A metal stripping process depends on the difference in chemical activity of the metal to be stripped and the substrate for the metal. For successful stripping the metal to be stripped must be active and the substrate relatively inert to the stripping solution. A solution can always be found that will dissolve a coating on a substrate but only a very few solutions will dissolve a metal coating without affecting the substrate. Alkaline cyanide solutions are used to electrostrip (anodically dissolve) metals and acids are generally used for immersion stripping of metals from a substrate. Both of these methods are difficult to carry out without affecting the substrate, especially if the substrate is steel.
A metal strip solution comprising an alkaline cyanide and an organic acid stripping agent, such as, benzenesulfonic acid-3 nitro, has been used successfully for immersion stripping cadmium, zinc, tin, copper, nickel, silver and other metals from steel substrates without affecting the steel substrate. Stripping of metals occurs rapidly without an anodic potential at low temperatures; however, the strip solution becomes saturated with dissolved metals and the spent solution must now be treated as a toxic waste.
Heretofore, there has been no method for restoring the used alkali cyanide-organic acid metal strip solution so that it can be used again and thereby eliminate the hazardous and costly treatment of the spent strip solution for disposal as a waste.
Attempts to restore the spent strip solution using electrodialysis, ion exchange and other non-destructive chemical methods have been unsuccessful because the metals are chelates or complex compounds. Metals are electroplated from alkaline cyanide solutions in conventional electrolysis cells having a soluble metal anode that does not form oxygen in the plating liquor and a cathode for electrodepositing metals. In this conventional process, metals are stripped (anodically dissolved) at the anode at essentially the same rate as metals are deposited at the cathode and this conventional process with a soluble metal anode is not suitable for removing metals from a spent metal strip solution of alkaline cyanide, organic metal stripping agent and dissolved metals. Attempts to use insoluble anodes to electroplate metals from the spent strip solution resulted in decomposition of the strip solution.
Disposal of the spent strip solution has required chemical destruction and removal of metals from the solution. Chemical destruction is generally carried out using excess calcium or sodium hypochlorite and several hours are required to decompose the solution for metal salt removal. Most on-site waste treatment facilities are inadequate for treating the spent strip solution and the solution must be shipped to an off-site waste treatment facility.
It would be desirable to restore the spent strip solution without producing a toxic waste and restore the solution on-site of use to reduce potential hazards of handling and shipping. Preferably, the strip solution would be restored continuously as used without producing a toxic waste so that the quantity of strip solution would be a minimum. It is an object of the instant invention to provide a process capable of continuous restoration of metal strip solutions comprising alkaline cyanide and an organic metal stripping agent without producing a toxic waste whereby the strip solution can be used again and again for stripping metals and the stripped metals are recovered as metals for use in electroplating and other processes.